In the development of internal combustion engines, it is constantly sought to minimize fuel consumption and reduce emissions. The low fuel consumption of a direct-injection diesel engine results inter alia from the direct injection of the fuel into the combustion chamber of the at least one cylinder. The injection of fuel directly into the combustion chamber of the cylinder is therefore considered to be a suitable measure for noticeably reducing fuel consumption even in spark-ignition engines. A direct-injection, spark-ignition internal combustion engine is also the subject matter of the present disclosure.
The non-homogeneity of the fuel-air mixture is also the reason why the particle emissions from the diesel engine process are likewise of relevance in the case of the direct-injection, spark-ignition engine, whereas said emissions are of almost no significance in the case of the traditional, spark-ignition engine. In the case of the direct injection of fuel, problems are caused by the coking of the injection device.
Methods in which the fuel is introduced into the combustion chamber of the at least one cylinder by a plurality of injections duly eliminate the wetting of the combustion chamber interior wall with liquid fuel, and thus contribute to a reduction in particle emissions. Such methods, however, lead, in principle, to an increase in the total number of injections, which promotes the deposition of coking residues on the injection device. Extremely small quantities of fuel which adhere to the injection device during the injection undergo incomplete combustion under oxygen-deficient conditions.
Although said coking residues do not, necessarily, change the geometry of the injection device, nor do they adversely affect through flow characteristics and/or hinder the formation of the injection jet thereby disrupting the mixture preparation, the deposits on the injection device do lead to increased particle emissions of the internal combustion engine. Injected fuel accumulates in the porous coking residues. Often toward the end of the combustion cycle, when the oxygen has been almost completely consumed, accumulated fuel undergoes incomplete combustion and forms soot. Soot contributes to the increase in untreated particle emissions of the internal combustion engine.
Coking residues may also become detached from the injection device for example as a result of mechanical loading caused by a pressure wave propagating in the combustion chamber or the action of the injection jet. The residues detached in this way not only increase the untreated particle emissions of the internal combustion engine but may also lead to damage in the exhaust-gas discharge system, and can impair the functional capability of exhaust-gas after treatment systems provided in the exhaust-gas discharge system.
Known from the prior art are concepts which are intended to counteract the build-up of coking residues and/or which serve to deplete deposits of coking residues, that is to say to remove said coking residues from, and clean, the combustion chamber. Additionally, manual methods of cleaning the injection device by ultrasonic washing exist, but no robust method is known to clean the injection device when assembled to the engine.
The German laid-open specification DE 199 45 813 A1 describes a method for operating a direct-injection internal combustion engine in a targeted manner for cleaning the combustion chamber following detection of deposits in the combustion chamber. Measures proposed for cleaning the combustion chamber include the targeted initiation of knocking combustion and/or the introduction of a cleaning fluid into the intake combustion air. Both measures have negative implications with regard to fuel consumption and pollutant emissions.
One cleaning fluid is water. The injection of which can lower the combustion temperature and the emissions of nitrogen oxides (NOx). The oxidation of soot however requires high temperatures. When using water, the coking residues are at best detached, but are not burned, resulting in an increase, rather than decrease, of the untreated particle emissions of the internal combustion engine. Furthermore, there is the risk of corrosion in the combustion chamber and in the exhaust-gas discharge system, and associated disadvantages as wear occurs.
The European patent EP 1 404 955 B1 describes an internal combustion engine whose at least one combustion chamber has, at least in regions, a catalytic coating on the surface for the purpose of oxidation of coking residues. The catalytic layer is intended to promote the oxidation of coking residues, specifically to promote fast oxidation, at typical operating temperatures, of the carbon-containing lining at a boundary surface between the catalytic converter and lining. This oxidation results in an early detachment of the deposit under the action of the prevailing flow. In this way, growth of the residues is reduced or even completely prevented. However, the minimum temperatures required for the oxidation are not always reached, even when catalytic materials are used. In particular, the required temperatures cannot always be attained at low loads and low rotational speeds. It is, however, precisely these operating conditions of the internal combustion engine, specifically low loads and/or low rotational speeds that promote the formation of coking residue deposits. Furthermore, the oxidation of the coking residues, that is to say of soot, requires not only adequately high temperatures but also oxygen or an excess of oxygen.
Another approach to adhere to future limit values for soot emissions would be for spark-ignition engines to be equipped with regenerative particle filters. However, such approaches are costly and can increase backpressure and reduce fuel economy.
The inventors herein have recognized the above issues and taken a different approach to address soot emissions of a direct injection spark-ignition gasoline engine, without reliance on a particulate filter. For example, one example method includes a method of operating a direct-injected, spark-ignited engine, comprising: direct-injecting gasoline to an engine cylinder; spark-igniting the injected gasoline; and responsive to an injector coking level and sufficiently high cylinder temperature, operating the at least one cylinder superstoichiometrically for a duration to increase oxidation of coking residues. In this way, excess oxygen is provided in the combustion process at precisely the conditions where the coking can be reduced, thus enabling reduced soot emissions while reducing the impact on three-way catalyst activity to a limited set of conditions. Specifically, because the oxidation of coking residues requires certain minimum temperatures in addition to an excess of air, the above approach only provides the lean combustion under limited conditions that are tied to the level of coking, for example. The method according to the disclosure significantly reduces the untreated particle emissions of a spark-ignition, direct-injection internal combustion engine. The untreated particle emissions may possibly be reduced to such an extent that exhaust-gas after treatment by means of a particle filter is not necessary for adhering to the legal limit values, although such a filter may be additionally used, if desired.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.